Add custom activity trails to Orders

[KevinFerm]

Is your feature request related to a problem? Please describe.
When I implement integrations like order exporters it would be nice to be able to push things to the activity log on a specific order.

Describe the solution you'd like
An api like oder.AddActvitiyLog() maybe

Describe alternatives you've considered
Using external systems, however it's not so visible for potential users of the backoffice.

[mattbrailsford]

You can use IActivityLogService.LogActivity to write to the log table, but I think we also have some front end stuff you need to do to be able to display them nightly. That depends on the version of Umbraco Commerce you are using though.